In the manufacture of a non-woven web, it is important to be able to determine certain properties continuously or intermittently. For example, it may be necessary to determine the thickness, mass, and/or density of the material at various stages of the manufacture of the web. Such measurement is also useful in other areas, such as in the food and paper industries.
It is well known that beta particles will be absorbed in a material as a function of the thickness or density of that material. The absorption is described by the Beer-Lambert Law, which is discussed below. In the prior art, certain pure beta ray emitters have been used to measure the thickness of certain materials. However, to make such measurements, a beta plus gamma emitter has not been used with two detectors, one of which detects a first type of radiation (for example, gamma radiation) and the other of which detects that first type and a second type (for example, beta radiation). It is desirable to be able to use a beta plus gamma emitter, for example Eu-154, to make measurements of density or thickness of material because such a beta plus gamma emitter emits relatively high energy beta rays and has a long half-life and, hence, can be used in measuring relatively dense materials and can be used without frequent source replacement.
The present invention enables one to use, for example, a beta plus gamma emitter for measuring mass, density, or thickness of materials including for example, a fabric made from polypropylene fibers, heavy gauge plastic film, paper, films of food, and certain light gauge metals (foils).
It is an object of this invention to measure the thickness and/or density of a material. Another object of this invention is to provide an apparatus for making such measurements.